[21.08] HST Observations of the Narrow-Line Regions and Broad-Line Regions of LINERs

Ground-based optical studies of low-luminosity AGNs face the
problem that the nuclear continuum light observed from the
ground is dominated by starlight, making it difficult to
detect nonstellar continua and broad lines. Small-aperture
spectra taken with the STIS spectrograph on HST can largely
circumvent this problem. We have obtained STIS optical
spectra covering 6300-6860 A of the nuclei of six low- to
moderate-luminosity, broad-lined AGNs of Seyfert and LINER
type. In NGC 4579, the spectra reveal a remarkable broad
H\alpha emission line with a full-width near zero
intensity of 18,000 km/s, which may originate from an
accretion disk. This very broad line may be a transient
feature, but it could have gone undetected in previous
ground-based spectra because of its low amplitude relative
to the continuum.

The spectra also reveal new structure in the narrow-line
regions. In the LINERs NGC 4579 and NGC 1052, the ratio of
[S II] 6717,6731 shows a pronounced gradient indicating a
steep rise in density toward the nucleus. The ratio [O I]
6300 / [S II] 6717, 6731 indicates a likely nebular density
of \gtrsim105 cm-3 at the nuclei. Such high-density
clouds are consistent with expectations from previous
observations of linewidth-critical density correlations in
AGNs. The direct detection of high-density clouds in these
LINERs supports photoionization as the dominant ionization
mechanism in the narrow-line gas.